Insights into the ecology and evolutionary history of the bacterial genus Psychrobacter

Abstract

The genus Psychrobacter presents an unusually broad range of isolation sources, ranging from the bodies of mammals to extremely cold and saline permafrost soils. In this thesis, I present my investigation of the diversity of Psychrobacter with the goal of identifying differences within the genus which may contribute to its surprising ecological distribution. I collected 85 Psychrobacter strains from diverse locations and isolation sources, and approached this problem using genomics, including phylogenomics and pan-genome analysis, as well as ancestral character estimation and analysis of protein coding sequences. I also performed extensive phenotyping, collecting growth curves in 24 different conditions in a gradient of temperature, salt, and nutritional complexity, as well as information on strain bile resistance, oxygen utilization, and for a subset of strains, ability to colonize germ-free mice. I show that Psychrobacter most likely evolved from a Moraxella-like ancestor, that is, a mesophilic pathobiont of mammals, and subsequently diverged into strains that maintain the ability to grow at high temperatures (ie 37 oC) versus those that only grow at cooler temperatures (25 oC or lower). There are some genomic and phenotypic differences amongst Psychrobacter strains based on source of isolation, but ultimately, dividing the genus by their behavior may be more useful. Overall, Psychrobacter promises to be an interesting model group for disentangling niche diversification among close relatives.